1. Field of the Invention
The invention concerns a miniature lightning protector, i.e. a miniaturized lightning protection device of the type which absorbs, by a short-circuiting electrical discharge, overloading caused by phenomena of atmospheric or other origins (lightning, inductions, accidental contact, etc..) and which may occur across the terminals of units, circuits or electrical or electronic installations, normally operating on low current, for example, subscriber telephone installations.
2. Description of Prior Art
The miniature lightning protectors, especially those intended for the protection of subscriber telephone installations, must conform relatively severe requirments concerning their operating characteristics and dimensions. Those most commonly used comprise two discharge electrodes, in a sealed box, which contains a radioactive gaseous atmosphere. The discharge is therefore absorbed in a short time, because of the high probability of the presence of at least one germ electron in the gap between the electrodes, at the time when an overload wave occurs.
These current types of miniature lightning protectors are normally mounted, when intended to protect subscriber telephone installations, in junction boxes which contain, integral with a case or a panel, five screw terminals, two removable fuses and two removable lightning protectors. Two of the terminals connect the box with the two line conductors connected to the network. Two others are connected to the two conductors of the subscriber installation and the last one is connected to a local ground. The two fuses are respectively connected between one of the network terminals and one of the installation terminals. The two lightning protectors, which are respectively connected between one of the line terminals and the ground terminal, are intended to shunt the line current to ground, in the case of a sudden overload, due, for instance, to a lightning strike. This type of junction box has several disadvantages. Its structure is heterogeneous and its arrangement is consequently complicated, its installation and removal take a relatively long time, and the protection which it provides is uncertain.
The structure of the said type of junction box is heterogeneous because it is made from an assembly of parts which must undergo very different mechanical or electrical stresses and which are consequently made of different materials. Without even mentioning insulating materials, when a box is installed, up to seven combinations of different types of alloy or metal could be mentioned (for example nickel-plated copper-brass, nickel-plated brass and cadmium-plated steel, cadmium-plated steel and stainless steel, etc..) resulting from contacts being made by clamping or brazing. Production is all the more complicated that the fuses and lightning protectors are separate interchangeable components, which are produced separately. The result of all this, among other unfavorable consequences, is an irregular resistance to corrosion and expensive manufacture.
The installation and removal are long, as they involve many different operations, including the fixing of the base of the case by screws and plugs on a building wall, the trimming and uncovering of the five conductors, their clamping by terminal screwing, installation of the fuses and lightning protectors and the fixing of the box cover. In addition, as a precautionary measure and especially in rural areas, the box should preferably be placed high on an outside wall, above the ground, so that it is impossible for a person to touch the box accidentally. This does not facilitate operations.
The protection provided is not sure for several reasons. The geometry of the components is unsuited to pulse operation. For example, a fuse wire and the plugs which are soldered onto it form a turn of a coil which, during high overload pulses, may be subjected to violent internal mechanical reactions, which may break the soldered joint, i.e. destroy the fuse, while the pulse itself was too short to be dangerous.
Generally, a correctly calculated fuse is also fragile due to the fact that the wire is very fine and made of silver-tin alloy, which is mechanically very weak. Therefore, in order to limit the service engineers' traveling, there is an unfortunate tendency to use oversize wire, especially in rural areas, i.e. exactly where the lines are the most exposed to lightning strikes. Thus, the fuses no longer fulfil their protective role.
In addition, it should be noted that the failure of a fuse is not shown on the switchboard. The failure can only be detected when the subscriber calls from another telephone, which may be far from his own.